The present invention generally relates to processing of materials for growth of crystals. More particularly, the present invention provides a semi-insulating gallium-containing nitride crystal synthesized by an ammonobasic or ammonoacidic technique. The present invention provides methods suitable for synthesis of polycrystalline nitride materials, as well as other crystals and materials. Such crystals and materials include, but are not limited to, GaN, AN, InN, InGaN, AlGaN, and AlInGaN, and for manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors.
Gallium nitride containing crystalline materials serve as substrates for manufacture of conventional optoelectronic devices, such as blue light emitting diodes and lasers. Such optoelectronic devices have been commonly manufactured on sapphire or silicon carbide substrates that differ in composition from the deposited nitride layers. In the conventional Metal-Organic Chemical Vapor Deposition (MOCVD) method, deposition of GaN is performed from ammonia and organometallic compounds in the gas phase. Although successful, conventional growth rates achieved make it difficult to provide a bulk layer of GaN material. Additionally, dislocation densities are also high and lead to poorer optoelectronic device performance.
Quality substrates comprising bulk gallium nitride are available commercially, however, in most cases, these substrates are electrically conductive. In some cases, a substrate that is electrically insulating or semi-insulating is desirable. In addition, bulk gallium nitride substrates are generally expensive, and substrate diameters of 2 inches and larger are only available with a c-plane orientation.
Several authors have disclosed the addition of transition metal deep acceptor dopants, e.g., Mn, Fe, Co, Ni, Cu, etc., to compensate donor species in the gallium nitride, and impart semi-insulating character to the gallium nitride. For example, Monemar and Lagerstedt [J. Appl. Phys. 50, 6480 (1979)] added Fe or Cr to GaN grown by hydride vapor phase epitaxy (HVPE) and obtained highly resistive crystals. Heikman et al. [Appl. Phys. Lett. 81, 439 (2002)] introduced Fe into GaN films grown by metalorganic chemical vapor deposition (MOCVD) and similarly obtained semi-insulating character. Generally, these authors were not able to obtain high quality, free standing bulk GaN wafers.
U.S. Pat. No. 6,273,948, issued to Porowski et al., describes a method of fabricating highly resistive GaN bulk crystals by crystallization from a solution of atomic nitrogen in a molten mixture of gallium and Group II metal such as beryllium or calcium, under a high pressure of about 0.5-2.0 GPa and a high temperature of 1300-1700 degrees Celsius. A resistivity of 104 to 108 ohm-centimeter (ohm-cm) was achieved. The crystal obtained from the process was about 1 cm in size, whereas most commercial electronic applications require a substrate size of at least about 2 inches (>5 cm) diameter.
U.S. Pat. No. 7,170,095, issued to Vaudo et al., describes an improved HVPE method for doping free-standing GaN crystals with relatively high crystalline quality. The HVPE technique, however, generally produces bulk GaN crystals of relatively high cost. U.S. Pat. No. 7,078,731, issued to D'Evelyn et al., teaches an ammonothermal method for synthesizing semi-insulating GaN crystals, for example, by doping with Fe or Co. The Fe-doped and Co-doped GaN crystals, however, are reddish/amber or black in color, respectively, rather than transparent and colorless.
What is needed is a method for low-cost manufacturing of semi-insulating nitride materials that are transparent, colorless, and of high crystallographic quality.